Tuned cavities are widely used in microwave communications applications. Coaxial cavities, square prism filters, and cubical "multiple cavity" devices such as those disclosed in commonly owned U.S. Pat. No. 4,249,148, are commonly used to implement bandpass filters, notched filters, composite filters, and combiners. Temperature compensating such tuned cavities for thermal expansion and contraction of housings or internal resonators of tuned cavities so that their resonant frequencies remain constant as their temperatures vary has generally been accomplished by manufacturing the housings of a material commonly known as Invar. Invar is a metallic compound having a very low positive temperature coefficient, and does not expand as temperature increases nearly as much as copper. However, the electrical conductivity of Invar is far too low for it to be satisfactory as the inner surface material of a tuned cavity, which inner surface must have an extremely high electrical conductivity to provide satisfactory performance. Therefore, temperature compensated square prism filters or cubic filters usually have their interior Invar surfaces gold plated in order to provide the required high conductivity. The necessity of providing gold plated inner surfaces obviously is very expensive. Furthermore, as the need for economical high "Q" tuned cavities has increased in the microwave art, it has been sometimes necessary to increase the volume, and thus, the interior surface area of tuned cavities, thereby increasing the cost of gold plating the interior surface area.
Coaxial tuned cavity devices have been made relatively temperature insensitive by using threaded Invar tuning rods in the construction of adjustable length, copper sleeve-type resonators, the length of which determines the resonant frequency. However, in some instances, even the slight change in length of the Invar tuning rods causes unacceptable variation in resonant frequency with temperature changes, and expensive, inconvenient techniques such as attaching the upper end of the Invar rod to a large bracket attached to the top of the cylindrical housing are used to physically counteract the temperature variation in the length of the Invar rod as the temperature changes.
Another approach to temperature compensating of tuned cavities that has been used is to provide a suitable bi-metallic coil on the outer surface of a tuned cavity so that a free end of the bi-metallic coil controls the amount of insertion of a conductive probe extending through the wall of the tuned cavity and into the internal volume thereof. As the temperature of the bi-metallic coil increases, it causes the conductor to tend to move out of the interior volume, decreasing capacitive loading of the resonant signal in the tuned cavity, thereby tending to offset the decrease in frequency caused by expansion of the frequency determining components (i.e., the walls of a square prism filter or the resonator of a coaxial filter) of the tuned cavity. However, this approach has been found to be unreliable, for two reasons. First, it has been very difficult to provide the needed high conductivity electrical path from the probe to the inner surface of the tuned cavity. Second, friction between the probe and the wall of the counter as the probe slides through a hole in the wall causes the inward and outward movement to be erratic, causing erratic and inaccurate temperature compensation.
Thus, there remains an unmet need for a simple, low cost, highly reliable, and accurate temperature compensated tuned cavity.
Accordingly, it is an object of the invention to provide an accurately temperature compensated tuned cavity that does not need to have its frequency determining parts composed of Invar metal.
It is another object of the invention to avoid the necessity of plating the interior of a tuned cavity device with gold or other high conductivity metal.
It is another object of the invention to provide a low cost, reliable temperature-stable microwave resonant device with temperature compensating elements located within the resonant device.
It is another object of the invention to provide an improved means for both calibrating and temperature compensating a tuned cavity device.